Recording sheets for the ink jet printing process available today do not have all the properties required, particularly there is an urgent need to improve ink absorptiveness, ink absorption rate, image quality, water fastness and light stability. A preferred embodiment of the invention relates to improved recording sheets having excellent image quality, high ink absorptiveness and high ink absorption rate. In particular ink receiving materials are sought where the images recorded thereon are resistant to rubbing on the surface and remain intact when in contact with water and should not fade when exposed to light.
Ink jet printing processes are of two types: continuous stream and drop-on-demand.
In continuous stream ink jet printing systems, ink is emitted in a continuous stream under pressure through a nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the nozzle. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electric static field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium.
In the non-continuous process, or the so called "drop-on-demand" systems, a droplet is expelled from a nozzle to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
The invention is directed towards recording sheets and coating compositions that may be used in both recording processes.
It is known that recording sheets for ink jet printing must meet a number of stringent demands. The printed image has to fulfill the following properties:
High resolution PA1 High colour density PA1 Good colour reproduction PA1 High resistance to rubbing PA1 Good water fastness PA1 High light stability PA1 1. The ink needs to be absorbed quickly into the recording material. PA1 2. The jetted ink droplets have to spread circularly on the recording material and have to form well defined edges. PA1 3. Dye diffusion in the recording material has to be low so that the diameter of the colour points is not increased more than necessary. PA1 4. An ink droplet is not allowed to interfere with a droplet deposited earlier nor should it blur it. PA1 5. The recording material needs to have a surface that gives high colour density and brilliance. PA1 6. The recording material has to show excellent physical properties before and after printing.
The following conditions have to be met to fulfill these goals:
These requirements are partially contradictory, for example a quick ink absorption rate often results in bad resistance to rubbing of the surface.
Starting from these requirements for a recording material ways are looked for that will result in images showing high colour density together with high resistance towards rubbing of the surface. The best properties are achieved with recording materials having coated an ink receiving layer on a support.
Patents EP 0,298,424 and EP 0,407,720, patent applications EP 0,622,244 and JP 60-245,588 describe ink receiving layers including as pigment aluminium oxide/hydroxide with a pseudo-bohemite structure. Pseudo-bohemite is an agglomerate of aluminium oxide/hydroxide of formula Al.sub.2 O.sub.3.n H.sub.2 O where n is from 1 to 1.5. Preferentially this aluminium oxide/hydroxide with a pseudo-bohemite structure is used in the form of its colloidal solution, because only in this way recording materials with superior image quality are obtained. It is well known that such colloidal solutions are only stable at low concentrations of the active ingredient. The storage stability of such colloidal solutions is low and storage conditions have to be tightly controlled.
Ink receiving layers prepared with aluminium oxide/hydroxide isolated in its solid form from its colloidal solution give images after ink jet printing with a reduced image quality compared with ink receiving layers prepared with the same aluminium oxide/hydroxide in the form of its colloidal solution.
Colloidal solutions of aluminium oxide/hydroxide may be prepared for example by neutralising aluminium salts followed by ageing as described in patent application JP 60-245,588. Another possibility is the hydrolysis of aluminium alkoxides as described for example in patent application EP 0,736,491.
Recording sheets for ink jet printing are described for example in patent applications EP 0,622,244 and EP 0,698,499 where the recording sheets comprise aluminium oxide/hydroxide containing titanium dioxide from 0.01% to 1.0% and where this modified aluminium oxide/hydroxide is used in its solid form.
The image quality of recording materials prepared in this way is however not sufficient for today's requirements.